1. Field of the Invention
The present invention relates to an analog buffer circuit, and more specifically, to an analog buffer circuit capable of compensating a threshold voltage variation of a transistor produced by Low Temperature Poly-Silicon (LTPS) processes.
2. Description of Prior Art
With a rapid development of display technology, novel and colorful monitors with high resolution, e.g., liquid crystal displays (LCDs), are indispensable components used in various electronic products such as monitors for notebook computers, personal digital assistants (PDAs), digital cameras, and projectors. The demand for the novel and colorful monitors has increased tremendously. A Low Temperature Poly-Silicon Liquid Crystal Display (LTPS LCD) panel, on account of high resolution demands, is widely applied to various electronic devices.
Referring to FIG. 1 showing a functional block diagram of a conventional liquid crystal display 10, the liquid crystal display 10 includes a liquid crystal panel 12, a gate driver 14, and a source driver 16. The liquid crystal panel 12 includes a plurality of pixels, each pixel having three pixel units 20 indicating three primary colors, red, green, and blue. For example, the liquid crystal display 12 with 1024 by 768 pixels contains 1024×768×3 pixel units 20. The gate driver 14 periodically outputs a scanning signal to turn on each transistor 22 of the pixel units 20 row by row, meanwhile, each pixel unit 20 is charged to a corresponding voltage based on a data signal from the source driver 16 via a corresponding data line 24, to show various gray levels. After a row of pixel units is finished to be charged, the gate driver 14 stops outputting the scanning signal to this row, and then outputs the scanning signal to turn on the transistors 22 of the pixel units of the next row. Sequentially, until all pixel units 20 of the liquid crystal panel 12 finish charging, and the gate driver 14 outputs the scanning signal to the first row again and repeats the above-mentioned mechanism.
In the conventional liquid crystal display, the gate driver 14 functions as a shift register. In other words, the gate driver 16 outputs a scanning signal to the liquid crystal display 12 at a fixed interval. For instance, a liquid crystal display 12 with 1024×768 pixels and its operating frequency with 60 Hz is provided, the display interval of each frame is about 16.67 ms(i.e., 1/60 second), such that an interval between two scanning signals applied on two row adjacent lines is about 21.7 μs (i.e., 16.67 ms/768). The pixel units 20 are charged and discharged by data voltage from the source driver 16 to show corresponding gray levels in the time period of 21.7 μs accordingly.
Referring to FIG. 2 illustrating an equivalent circuit diagram of the data line 24 and the source driver of FIG. 1, the source driver 16 comprises a digital-to-analog converter (DAC) 161 and an analog buffer 162. An equivalent circuit of each data line 24 is a combination of a data line load capacitor C and a resistor R. The DAC 161 is used for converting digital data signal voltage into analog data signal voltage to charge the load capacitor C through a bias current from the analog buffer 162, so that an alignment of liquid crystal molecules is adjusted to show various grey levels based on the analog data signal voltage. Actually, driving ability of the source driver 16 depends on output resistance of output stage and magnitude of the bias current, yet threshold voltages of transistors of the analog buffer 162 are greatly varied over a large swing voltage to degrade display quality, especially the LCD made by using LTPS processes. Therefore, it is necessary to produce an analog buffer circuit capable of compensating a threshold voltage variation of a transistor.